Communications with an object situated well above ground, such as an aircraft or a satellite platform, frequently requires an antenna system providing a true omni-directional electromagnetic radiation pattern, i.e. a substantially spherical pattern with substantially constant gain over 4.pi. steradians. A spherical pattern is required because of the need to communicate with multiple sites distributed at random locations around the space platform when: (1) it is not feasible to maneuver the antenna or platform to provide the desired antenna pattern; or (2) simultaneous communication with more than one site is required.
While acceptable hemispherical radiation patterns may be achieved from a single antenna element, spherical radiation from a single antenna element is not possible due to unavoidable asymmetry in the antenna feed structure. Realizing spherical coverage from a phased array of antenna elements may be theoretically possible but practical implementations will always result in non-uniform field pattern characteristics (nulls) due to interactions between the radiation patterns of individual elements. Field pattern uniformity further degrades in those cases where individual elements must be separated by multiple wavelengths due to physical constraints, such as might occur when antennas must be mounted on opposite sides of an airplane or satellite, or if wide bandwidths are involved. In practice, obscurations caused by aircraft portions (wings, empennage and the like) or space platform structures (solar power panels, booms and the like) mitigate against spherical coverage and favor an approach using multiple distributed antennas. It is therefore highly desirable to provide an antenna system in which the patterns of a plurality of individual antennae are combined in such a manner as to achieve a substantially uniform spherical radiation pattern about a structure located well above ground level.